Electronic devices as power converters for driving motors are mounted on hybrid motor vehicles or electric motor vehicles. An electronic device such as a power converter converts direct current power supplied from a battery into alternating current power in order to drive a motor and, in a reverse manner, convert alternating current power regenerated by the motor into direct current power in order to charge an electrical storage device. In this type of electronic devices, semiconductor elements that generate heat at a high temperature are accommodated in a case. It is therefore necessary to immerse and cool the electronic devices in cooling medium such as cooling water.
As one example of waterproof electronic devices, a structure is known in which a semiconductor device accommodating semiconductor elements is provided with heat dissipating fins and fixed to a case by sandwiching the device from above and below, respectively with an upper wall part of the case body and a bottom plate of the case. In this waterproof electronic device, a space defined by a side surface of the semiconductor device and an inner wall of the case forms a flow path for cooling of the semiconductor device, and the semiconductor device is immersed in the coolant (see PTL 1).